This invention relates to surfactant flooding fluids for the recovery of petroleum from porous subterranean reservoirs.
The petroleum industry has long recognized that only a fraction of the original oil in a reservoir is expelled by natural mechanisms or primary production. Accordingly, there is a need for improved methods of increasing the ultimate yield of petroleum from natural reservoirs. Waterflooding is the most widely practiced secondary recovery method. In this method various additives can be added to the flood water to increase oil displacement. For instance a surfactant such as a petroleum sulfonate may be added to the water to lower the interfacial tension between oil and water. If enough surface active ingredient is added to lower the interfacial tension sufficiently, then more oil can be displayed from a rock by the water containing the surfactant than can be displaced by water not containing a surface active agent. If addition of a surface active agent lowers the interfacial tension to a value of 0.01 dynes per centimeter or lower, then the water containing the surface active agent will displace essentially all of the oil from most rocks. In contrast water not containing a surfactant will leave an oil saturation in the rock usually between 15 and 50 percent of the pore volume. Thus, the purpose of adding a surfactant such as a petroleum sulfonate to water is to increase the microscopic displacement of oil from the volume of the reservoir rock contacted by the water.
Another additive commonly employed in such fluids is a polymer. The purpose of adding polymer to the surfactant solution or dispersion is to decrease the mobility of the solution in the rock. This increases the volumetric sweep of the surfactant solution or in other words allows the solution to contact a much larger volume of the rock than it would without the polymer. The polymer also enhances the formation of an oil bank in front of the surfactant slug. Thus both surfactant and polymer are required to ensure both good microscopic and macroscopic (or volumetric) displacement of oil.
If a stable surfactant solution in which the surfactant will not precipitate with time is injected into a formation, the depth of penetration of the surfactant will depend upon the adsorption of the surfactant on the rock surface. If an unstable surfactant solution in which the surfactant will precipitate with time is injected into a formation, the depth of penetration of the surfactant will be less than that of a stable surfactant solution because the apparent adsorption will be higher due to precipitation of the surfactant.